In recent years, the use of smaller, inexpensive laser printers with personal computers has created an entirely new industry referred to as desktop publishing. Desktop publishing systems offer user the ability to format and print documents having complicated layouts using characters that have a variety of different fonts and type sizes. While desktop publishing systems represent a significant advance in the art of publishing, the standard resolution of the laser printers used with such systems (typically a 300.times.300 dpi Canon CX or SX-based laser printer, e.g. a HP LaserJet Series II printer) was too poor to compete with traditional phototypesetting systems.
In an effort to improve the quality and speed of the smaller, inexpensive laser printers used with desktop publishing systems, a variety of printer controller cards have been introduced that may be installed either in the laser printer or in the personal computer. Certain of the printer controller cards that are installed directly in the personal computer increase the quality and speed of the laser printers by using a separate co-processor and page frame buffer to create a pixel representation of the image to be printed on the laser printer. This image is then printed from the pixel representation in the page frame buffer by directly controlling the modulation of the laser in the print-engine of the laser printer. An example of such a printer control card is the LX6 Professional printer controller cards available from LaserMaster Corporation, Eden Prairie, Minn., the assignee of the present invention.
In some of the prior art printer controller cards, such as the LX6 Professional, it is possible to increase the horizontal resolution of the laser printer by controlling the modulation of the laser in the print-engine of the laser printer. By doing so, the LX6 Professional printer controller cards can increase the horizontal component of the pixel resolution of the laser printer to 600.times.300 dpi and 1000.times.400 dpi, depending upon the type of laser printer. This increase in horizontal resolution significantly improves the quality of the resulting printed image, particularly for pixel transition points along the outline edges of a character or image that occur in the vertical or near vertical orientation. Unfortunately, the increased horizontal resolution does little to improve the quality of the outline edges of a character or image that are near horizontal in slope.
The noticeability and appearance of such vertical pixel transition points is typically referred to as aliasing that results in jagged or stairstep edges of the character or image outline oriented generally in the horizontal direction. Traditionally, anti-aliasing techniques incorporating a gray-scale approach have been used in video displays to resolve this type of problem. Unfortunately, laser printers are binary imaging devices and are not capable of implementing such gray-scale techniques. In larger and more expensive laser printers, the problems associated with aliasing and non-smooth edges may be resolved by using higher pixel resolutions in these printers. In the smaller, inexpensive laser printers used with desktop publishing systems, this approach is not used because of the associated increase in cost.
While the prior art printer controller cards have allowed users of desktop publishing systems to significantly increase the quality of their printed outputs by increasing the horizontal resolution of the existing laser printers used with such systems, it would be advantageous to be able to improve the quality of the outline edges of a character or image that occur in a near horizontal orientation, particularly if such an improvement could be accomplished without any additional hardware that would increase the cost of such printer controller cards.